Self-tolerance of mature, peripheral CD4+ T cells in Vb5 transgenic (Tg) mice occurs through two pathways, deletion and T cell receptor (TCR) revision. Both processes are driven by chronic encounter with a peripherally expressed self-antigen encoded by mouse mammary tumor virus (Mtv) 8. Recognition of Mtv-8 expressed by dendritic cells renders CD4+Vb5+ T cells anergic and results in their deletion and the concomitant inversion of the peripheral CD4:CD8 ratio. T cells that chronically recognize Mtv-8 on B cells instead enter the germinal center and undergo TCR revision. During the process of TCR revision, T cells lose Vb5 surface expression, upregulate expression of the lymphocyte-specific recombinase machinery (RAG1, RAG2, and TdT), and rearrange endogenous TCR Vb genes. Deletion of Rag in peripheral T cells blocks TCR revision. Post revision T cells are Vb5- and fail to recognize Mtv-8, yet express a diverse TCRb repertoire and are self-MHC restricted and functional. The goal of this exploratory grant is to place the population of revising T cells in the context of a defined T cell subpopulation of known function. Our previous characterization of T cells in the process of TCR revision hint at a phenotypic overlap with the follicular helper T cell compartment. With this pilot grant, we will determine the relationship between the frequencies of revising T cells and germinal center B and T cells, and determine whether revising T cells bear the phenotypic and functional characteristics of follicular helper T cells. These experiments will help place TCR revision within the larger framework of T cell immunology, and will explore how TCR revision may impact the B cell arm of the adaptive immune response.